The mobile communications market is very competitive, and manufacturers strive to produce mobile communication devices with better, easier to use features, as well as improving operation and performance of these devices. One aspect of mobile communication device operation that is considered a key marketing feature is the so called “talk time.” This is the amount of time the mobile communication device can operate while transmitting before the battery needs to be changed or recharged. Generally, while a mobile communication device is transmitting, it uses substantially more power than in an idle state. That is, while the user of the device is engaged in a call, the device requires more power than when it is not engaged in a call. Accordingly, increasing the efficiency of the transmitter can result in significant increases in talk time.
Transmitter design is driven by the type of communications, or the air interface for which the mobile communication device will be used, and more specifically the type of modulation used. Some systems use linear modulation to reduce the bandwidth needed for channels. While bandwidth efficiency is an advantage of linear modulation, amplifier efficiency is a significant concern. In a typical transmitter using a radio frequency power amplifier having a fixed power supply voltage, as the input signal magnitude drops, so does the efficiency. The best efficiency is achieved when the RFPA is operating near saturation.
To keep the amplifier operating near saturation, some amplifier schemes us an envelope tracking scheme to modulate the voltage supplied to the RFPA in correspondence with the envelope of the signal being transmitted. To prevent starvation of the amplifier, some margin is designed into the scheme. However, adding margin to the supply voltage is less than ideal because it reduces average efficiency. At the same time, not having the margin will often cause distortion in the amplified signal because of unexpected gain compression. Therefore there is a need for means by which the supply voltage an be modulated without the worry of compression distortion, while maintaining optimum efficiency.